1. Field of the Invention
The invention relates to the field of mechanisms for making and breaking electrical circuits, and in particular concerns a mechanism that drives a circuit interrupter between open and closed positions and that has a resiliently yielding connecting rod for maintaining a predetermined closing pressure as the contacts of the interrupter erode with use.
2. Prior Art
Drive assemblies are known that transmit the drive force of an external drive mechanism to bring together or to separate paired contacts forming a circuit interrupter. A circuit interrupter for a single conductor ordinarily comprises a stationary contact and a movable contact. The mechanical force for driving the interrupter typically is supplied via a rotatable drive shaft, the motion of which is arranged to relatively displace the contacts linearly, e.g., along a longitudinal axis of opposed conductor stubs that meet endwise. A plurality of interrupters can be coupled to the same drive shaft or connected by linkages, for gang operation in which the contacts for a number of conductors are opened or closed as a unit.
In a typical drive assembly, rotation of the drive shaft through part of a revolution drives the movable contact linearly into and out of engagement with the stationary contact. In this manner the circuit interrupter is respectively closed and opened. Known interrupter drive assemblies have certain problems, particularly when the circuit interrupters are used in demanding applications. An example is a circuit interrupter used in an electrical power substation. A circuit interrupter for a substation may typically carry a line voltage of 15 KV. The line has an inductance, and can be coupled to various inductive loads. When the contacts are closed, electric current is free to flow across the engagement between the movable and stationary contacts. However, the flow of current across the contacts produces a repelling force that acts to urge the contacts away from one another. It is known, for example, that the movable and stationary contacts for circuit interrupters employed in 15 KV substations should be mechanically retained in engagement by virtue of a positive static load of 900 or so pounds, to overcome the current-induced repulsion between the contacts. This is a substantial load, and complicates the structure of the mechanism needed to make and break the contacts.
The need to include a biasing force for holding the contacts together, and the structure needed to do this, are subject to another problem involving the erosion of the surfaces of the contacts due to arcing. In air, contact surfaces erode during each high current interruption as a plasma arc bridges across the opening space between the contacts. It is possible to confine the contacts in a vacuum receptacle to reduce the arcing problem. However, some erosion of the contact surfaces still occurs. The interrupter is advantageously designed to be reusable for making and breaking the circuit over many repetitions. After repeated interruptions, however, erosion shortens the contacts to the point that they need to be repaired or replaced for continued dependable operation. For example, in a typical structure after the contacts lose about 3 millimeters or so from their surfaces as measured in the direction of relative advance or retraction, they need to be replaced or the interrupter may not operate dependably to maintain the necessary static load.
The interrupter drive assembly is designed with a particular stroke length in mind. The erosion of the contact surfaces changes the dimensions of the apparatus, and affects the performance of the drive assembly. The outer limits of travel of the drive mechanism are generally limited by mechanical stops, and the retracted position of the contacts may also be limited by a mechanical stop. However, the advanced position of the contacts varies with their erosion. Thus, for example, the angular displacement of the drive shaft encompasses a full preset range and does not vary. With the circuit interrupter contact, however, a contact structure that was arranged to travel back and forth about 14 millimeters or so relative to the stationary contact when new, may have to travel 17 to 20 mm between the point at which its surface bears on the stationary contact and the retracted position of the movable contact, normally fixed by the structure.
Thus, the reversible input motion to the drive assembly has a set stroke, but the reversible output motion applied to the contacts must vary over time to account for the additional gap length due to contact erosion. One of the design objects of an interrupter drive assembly is to compensate for the variance between the relatively unchanging input stroke and the need for a gradually increased output stroke. This must be done while retaining a structure that will apply a substantial force to the contacts when they are closed, to overcome electromagnetic repulsion.
It would be desirable to improve on known interrupter drive assemblies to find an optimal solution to the contact pressure and changing stroke length problems. Such a device advantageously would maintain a predetermined static mechanical load on the movable contact when the movable contact is engaged against the stationary contact, which load does not decrease substantially due to a change of contact stroke length. Therefore, the device should maintain the load in a manner that compensates for the increasing variance between the input and output motion as the contact surfaces erode away. These requirements could advantageously be provided in an improved drive assembly characterized by better performance, simplicity, and reliability than known interrupter assemblies.